Prostate cancer is the most common form of non-skin cancer in US men (1). The lifetime risk for developing prostate cancer is approximately one in six in the US (2). Morbidity and mortality due to prostate cancer are likely to grow further given increasing longevity. Epidemiologic studies indicate that infection and inflammation may play a role in the development of prostate cancer (3, 4). A recent search for viral nucleic acids in prostate cancer tissues led to the identification of a novel retrovirus, Xenotropic murine leukemia virus-related virus (XMRV) in approximately 10% of prostate cancers tested (5). XMRV, by sequence analysis, is a gammaretrovirus. While gammaretroviruses have well- characterized oncogenic effects in animals, they have not been shown to cause human cancers. Our preliminary results show that XMRV is indeed a gammaretrovirus with protein composition and particle ultrastructure highly similar to another gammaretrovirus, Moloney murine leukemia virus (MoMLV). We analyzed 334 consecutive prostate resection specimens using a quantitative PCR assay and immunohistochemistry with an anti-XMRV specific antiserum. We found XMRV in 27% of prostate cancers and in only 6% of non-cancer controls, showing for the first time an association of XMRV with prostate cancer. XMRV proteins were expressed primarily in malignant epithelial cells, suggesting that retroviral infection may be directly linked to tumorigenesis. The presence of XMRV correlated with higher-grades of prostate cancers. We found XMRV infection to be unrelated to a common polymorphism in the RNASEL gene, unlike previously reported. This increases the population at risk for XMRV infection from only those homozygous for the RNASEL variant, to all individuals. Our goal is to further understand pathogenesis by XMRV, to determine if this virus causes prostate cancer, and delineate possible mechanisms for oncogenesis. We will also carry out epidemiological studies to study the prevalence and tropism of this new and important virus. Our specific aims are: Aim 1: Characterization of XMRV replication in cell culture; testing mechanisms of oncogenesis: We will use the full- length replication-competent DNA clone of XMRV that we have assembled, and the polyclonal XMRV-specific antisera we have generated to characterize different steps in the viral life cycle. Aim 2: Further characterization of XMRV infection in human prostate cancers. We will continue our analysis of large numbers of de-identified human prostate cancers. We will further characterize the presence and localization of XMRV within cancers of different grades, stages and histologic subtypes and its association with different prognostic features of prostate cancer. Aim 3: Epidemiological studies to look for prevalence of XMRV in the general population: in blood, and in tissues other than prostate. Our studies could causally link XMRV to carcinogenesis and lead to novel modalities for cancer prevention and treatment.